1. Field of the Invention
The present invention relates to an ionosonde having a function to find the directions of arriving radio waves and particularly to an ionosonde performing direction finding based on not the directionality of an antenna, but the analysis of electric and magnetic fields of the arriving radio waves.
2. Description of the Prior Art
Ionosondes are ionospheric sounding devices that transmit to the sky pulsed radio waves whose frequencies are continuously scanned, receive on the ground the radio waves reflected by an ionosphere and prepare an ionogram showing the distribution of ionized gas based on the time of delay of the radio waves reflected by the ionosphere and returned to the ionosonde. Conventional ionosodes are not equipped with a direction-finding function having sufficient resolution, failing to obtain sufficient knowledge on the direction. For this reason, when examining the state of an ionosphere from an ionogram, analysis has been made on the assumption that radio waves transmitted to just above would be returned from just above after being reflected by the ionosphere. It has been known that this assumption is not materialized where the ionosphere is slanted or irregularly disturbed, or has a fine structure, but is correct where the ionosphere is spread flat. Where the assumption is not materialized, as described above, if the directions from which the individual pulsed radio waves transmitted to just above are returned are found, the ionogram can strictly be interpreted, enabling more exact examination of the state of the ionosphere.
In order to find the directions of the arriving radio waves with conventional ionosondes, Adcock antennas or crossed-loop antennas have been used. In short wavebands used for ionospheric observation, however, the azimuth error of the ionosonde is some tens of degrees insufficient for obtaining knowledge on the azimuth components in ionospheric distribution.
As an ionosonde having a direction-finding function is reported Digisonde in Radio Science Vol. 13, No. 3, pp 519-130, 1978. The Digisonde uses ten crossed-loop antennas to find the direction from which individually transmitted radio waves are returned. The Digisonde is similar in the aspect using loop antennas to, but different in the aspect using antenna directionality to obtain directional signals from the present invention. The azimuth resolution obtained through synthesis of the directionality of the loop antennas with the Digisonde is about 30 degrees insufficient for examining the structure of an ionosphere.
As described above, though the conventional Digisonde having a direction-finding function uses the directionality of the antenna, the azimuth resolution in the short waveband for an ionosonde is some tens of degrees insufficient for examining the ionospheric structure.
An object of the present invention is to provide an ionosonde having a direction-finding function that can observe with high-precision resolution the direction receiving the reflected waves from an ionosphere.
To attain the above object, an ionosonde according to the present invention comprises a signal-transmitting section for transmitting to an ionosphere pulsed radio waves whose frequencies are scanned; a signal-receiving section comprising a loop antenna system for measuring intensity of a magnetic field Hz of pulsed radio waves reflected by the ionosphere, a first dipole antenna system for measuring intensity of an electric field Ex orthogonal to the magnetic field Hz and a second dipole antenna system for measuring intensity of an electric field Ey orthogonal to the magnetic field Hz and electric field Ex; and a signal-processing section using three intensity values obtained at the signal-receiving section to obtain a cosine (nx, ny) of an arrival direction {right arrow over (n)} of the reflected radio waves in accordance with relationship nx=(HzEx*xe2x88x92ExHz*)Z/(ExEy*xe2x88x92EyEx*) and relationship ny=(HzEy*xe2x88x92EyHz*)Z/(ExEy*xe2x88x92EyEx*), wherein Z stands for intrinsic impedance.
The above object can also be attained by an ionosonde comprising a signal-transmitting section for transmitting to an ionosphere pulsed radio waves whose frequencies are scanned; a signal-receiving section comprising a dipole antenna system for measuring intensity of an electric field Ez of pulsed radio waves reflected by the ionosphere, a first loop antenna system for measuring intensity of a magnetic field Hx orthogonal to the electric field Ez and a second loop antenna system for measuring intensity of a magnetic field Hy orthogonal to the electric field Ez and magnetic field Hx; and a signal-processing section using three intensity values obtained at the signal-receiving section to obtain a cosine (nx, ny) of an arrival direction {right arrow over (n)} of the reflected radio waves in accordance with relationship nx=xe2x88x92(EzHx*xe2x88x92HxEz*)/(HxHy*xe2x88x92HyHx*)Z and relationship ny=xe2x88x92(EzHy*xe2x88x92HyEz*)/(HxHy*xe2x88x92HyHx*)Z, wherein Z stands for intrinsic impedance.
In each of the ionosondes, the signal-receiving section is not less than 10 km distant from the signal-transmitting section.
In order to obtain ionograms with few noises, each of the ionosondes can further comprise a display section for displaying or accumulating three values measured in respect of electric and magnetic field intensities of the pulsed radio waves transmitted from the signal-transmitting section and then received by the signal-receiving section.
The display section also indicates the arrival directions of the reflected radio waves and at that time also displays directgrams as well as the Monograms.
The display section can display directgrams with a plurality of colors, the state of directions receiving the reflected radio waves with hues, chroma and brightness changed, and ionograms with predetermined hues, chroma and brightness.
As described above, the ionosonde of the present invention performs direction finding based on not the directionality of an antenna, but the analysis of three values (each composed of an amplitude and a phase) obtained by measurement of electric and magnetic fields of the arriving radio waves, i.e. one magnetic field in an optional direction and two electric fields orthogonal to the optional direction, or one electric field in an optional direction and two magnetic fields orthogonal to the optional direction.
The ionosphere-observing waves that are artificial radio waves generally have larger intensity and more stable phase than radio wave noise. For this reason, they have good direction precision at short wavebands, specifically a direction error as small as approximately three.
With each of the ionosondes, the display section can display ionograms and directgrams with a plurality of colors, and different hues and chroma. This enables the directions of the radio waves reflected by the ionosphere to be grasped with ease.